PROJECT SUMMARY Relevance. Leishmania parasites infect an estimated 12 million individuals, and no satisfactory treatments are yet available. This proposal will advance 3 scaffolds that have emerged from a phenotypic screen for development of leads towards novel effective orally-bioavailable drugs against leishmaniasis. Summary. A previous phenotypic screen against the disease-causing intracellular amastigote stage of Leishmania parasites has identified 3 chemical scaffolds with potent efficacy against amastigotes. Each hit compound exhibits nanomolar efficacy against amastigotes growing in tissue culture macrophages and has physical and chemical properties suggesting promise for development of a novel orally bioavailable drug. Analogs of each scaffold will be prepared and tested for efficacy against amastigotes in vitro, optimal absorption/distribution/metabolism/excretion/toxicity (ADMET) profiles, acceptable in vivo pharmacokinetics (PK), and efficacy in controlling disease in a murine model of visceral leishmaniasis. Following establishment of initial structure-activity relationships, an extensive medicinal chemistry program will be undertaken to develop analogs with the best overall properties for orally bioavailable drugs and to improve upon any potential deficits that emerge in the original hits. The efficacy of each scaffold against multiple species of Leishmania and against drug resistant field isolates will be tested at an early stage to ascertain the potential of each scaffold for applicability against a broad range of parasites causing distinct types of leishmaniasis and with or without pre-established resistance to other currently employed, albeit non-optimal, antileishmanial drugs. The rate of development of resistance will be monitored to ensure that leads that are pursued will be reasonably stable against emergence of resistance during use in the field. In association with an extensive medicinal chemistry program, in vivo pharmacokinetics and acute toxicity studies and ability to control visceral infections in mice and hamsters will be determined to arrive at the top synthetic leads originating from the original 3 hits. The overall objective is to develop several leads with highly promising characteristics for ultimate development of desperately needed novel and effective drugs against this widespread and poorly controlled parasitic disease.